1. Field of the Invention
The present invention relates to methods of manufacturing a silicon carbide semiconductor device, more particularly relates to a method of preparing a terminal structure of the silicon carbide semiconductor device.
2. Description of the Related Art
A silicon carbide semiconductor device that uses silicon carbide (SiC) for its substrate material is known as a semiconductor device excellent in breakdown voltage and temperature characteristics in comparison with a silicon semiconductor device that uses silicon (Si), a conventional substrate material. While various silicon carbide semiconductor devices have been proposed, many difficulties to be solved still exist against realization of a silicon carbide semiconductor device applicable to practical use. One of these difficulties is considerable lowering of breakdown voltage of semiconductor device due to an electric field concentration arising at, for example, the edge of a Schottky electrode in a Schottky barrier diode (SBD), or the edge of a PN junction in a PN diode or in a metal oxide semiconductor field effect transistor (MOSFET).
A terminal structure called “junction termination extension (JTE)” has been known as a structure for relaxing such an electric field concentration arising at the edge of a Schottky electrode or a PN junction.
The JTE is a p-type region (hereinafter, referred to as “JTE region”) that is provided extending from the edge of a Schottky electrode or of a PN junction heading toward its peripheral region, in which a concentration of impurities decreases stepwise. In other words, the JTE region is formed of a plurality of p-type portions with different impurity concentrations that decrease stepwise from the edge of the Schottky electrode or the PN junction toward its peripheral region. In this way, by forming a p-type region having such an impurity concentration decreasing stepwise toward its peripheral region from the edge of a Schottky electrode or a PN junction thereby to realize a wide range of impurity concentration in the region, a JTE region having a desired breakdown voltage has been obtained. See, for example, Japanese Patent Applications Publication No. 2006-165225A and International Patent Application Publication No. WO98/02924.
As described above, in order to form a JTE region having a desired breakdown voltage, a plurality of p-type portions needs to be formed with different impurity concentrations decreasing stepwise from the edge of a Schottky electrode or a PN junction toward its peripheral region. Forming such a plurality of p-type portions requires processing steps for the number of p-type portions to be formed. For example, at least three steps, i.e., a mask formation step, an ion-implantation step, and a mask removal step are needed to form one p-type portion. Thus, the process of these three steps is needed for the number of p-type portions to be formed.